This invention relates to a magnetic resonance imaging apparatus (hereinafter this is referred to MRI), especially to technology being suitable for measuring an image of a temperature distribution in a living body with a MRI apparatus.
Recently interventional MRI (hereinafter, this is referred to IV-MRI) using an MRI apparatus as a surgery monitor is attended. In a therapy method performed by using IV-MRI, there is a laser treatment, a drag injection of ethanol or the like, an RF irradiation excision, and a low temperature treatment. In these treatments, MRI plays a role for guiding a needle or a narrow tube to a diseased part by real time imaging, or visualizing a change of tissues in said treatments, or monitoring a limited part in application of heating or cooling treatment, or the like. A typical application for IV-MRI includes an imaging of the temperature distribution within the body when said laser treatment is performed.
Imaging method of the temperature distribution includes method for calculating from signal intensity, method for calculating from diffusion coefficient, method for calculating from phase shift of proton (PPS method: Proton Phase Shift method), or the like. PPS method is the best in measuring accuracy. This function has made it possible to monitor the temperature in the living body, monitor of the laser irradiating treatment, temperature monitor of the RF (Radio Frequency) ablation.
For example, in PPS method a temperature distribution is calculated from phase information of an echo signal obtained with alternating gradient magnetic field. FIG. 1 shows one example of a method to measure such phase information. Hereinafter, a measuring method shown FIG. 1 and a calculating method of phase distribution or temperature distribution will be described.
At first, a spin in a sample is excited by irradiating 90xc2x0 high frequency pulse 101 (hereinafter this is referred to 90xc2x0 pulse) together with an application of gradient magnetic field Gs 102 for selecting the slice. Then gradient magnetic field Gp 103 and reading out gradient magnetic field Gr 104 for changing the phase of excited spin are applied successively to generate gradient echo signal 105. Then a Fourier transformation is performed on said echo signal to create a real part or an imaginary part of complex image, and a phase distribution is calculated. For example, said phase distribution is calculated from equation (1).
"PHgr"(x,y,z)=tanxe2x88x921{Si(x,y,z)/Sr(x,y,z)}xe2x80x83xe2x80x83(1)
And temperature T is calculated from acquired phase distribution, interval TE (106) between the time that echo signal is the maximum and the time applying 90xc2x0 pulse, resonance frequency f, temperature coefficient of water. For example, temperature T is calculated from equation (2).
T[xc2x0 C.]="PHgr"[xc2x0 ]/{TE[s]*f[Hz]*0.01[ppm/xc2x0 C.]10xe2x88x926*360[xc2x0 ]}xe2x80x83xe2x80x83(2)
By using said method, the difference of the temperature distribution is calculated respectively from signal acquired at a different time t1 . . . tn (n is a imaging number) to produce the distribution of temperature change in the object to be examined at a certain time.
When images are acquired with the repetition of measurement, temperature of a magnet and a pole piece is rose with the heating-up of GC coil, and a static magnetic field and offset of the gradient magnetic field changes. Changing of the static magnetic field and offset of the gradient magnetic field is equivalent to the change of the spatial distribution of resonance frequency f. Therefore if resonance frequency f is not measured at every position of every imaging, accurate temperature distribution is not obtained because it is affected by the static magnetic field change. However, IV-IMR is necessary to acquire images of temperature distribution with real time. But measuring the distribution of resonance frequency f at every imaging, imaging time is increased. Then the real time in imaging is lost.
As thus described when the change of temperature distribution is measured with MRI apparatus, it is important to avoid influence caused by change of the static magnetic field for obtaining the accuracy or the real time of the temperature distribution.
The object of the present invention is to solve these problems and to provide high accurate image of temperature change distribution.
In order to solve said problems in the present invention, a magnetic resonance imaging apparatus comprises static magnetic field generation means for applying a static magnetic field to the object to be examined, gradient magnetic field generation means for applying a slice direction gradient magnetic field, a frequency encoding gradient magnetic field and a phase encoding gradient magnetic field to said object, high frequency pulse generation means for generating high a frequency pulse for causing magnetic resonance to the nucleus composing said object, detecting means for detecting a nuclear magnetic resonance signal from said object, reconstruction means for reconstructing an image from said a nuclear magnetic resonance signals, display means for displaying said reconstructed image, and a control means for controlling said each means. And said control means performs measuring spatial phase distributions including a temperature information at different time, getting a reference phase in said spatial phase distributions by complex difference calculating to said spatial phase distributions at said different time, calculating the difference between said reference phase and a spatial phase distribution applied to said difference calculation, and getting temperature change distribution from said spatial phase distribution applied to said difference calculation.
In addition, said magnetic resonance imaging apparatus further comprises selecting means for selecting arbitrary position in the image displayed on said displaying means. And it is preferable to get said reference phase from the position selected by said selecting means, and to select plural number of points which is positions for getting reference phase after specifying arbitrary region in the image displayed on the display means.
In addition, it is preferable to get reference phase from at least one point within the region where temperature is not changing or plural number of points that do not form a line within the region where temperature is not changing by said control means.
In addition, it is preferable to display in parallel temperature image including temperature information and reconstructed anatomical image, or superimpose image of temperature changing region in said temperature image including temperature information on the reconstructed anatomical image by said display means.
In addition, it is preferable to specify arbitrary region in the image displayed said display means and select plural number of points for calculating reference phase on the line in said region, and to specify region including temperature change region.
In addition, it is preferable to perform by fitting process considering temporal change of the reference phase.
In the present invention having previous described compositions, temperature distribution change is obtained accurately by correcting the change of spatial phase distribution at each position due to static magnetic field change with reference to the phase change of plural number of points on the region where temperature is not changing, for example arbitrary three points or plural number of points above line in the arbitrary region.